Identity thieves have figured out how to turn aging gas pumps into cash without even "breaking and entering," according to law enforcement.

When most gas pumps were manufactured decades ago, their locking inner compartments came with a single, universal key. That design was intended to let inspectors and maintenance crews gain easy access without having to ask each station owners for a fresh key with each visit.

But that was back when gas pumps were little more than conduits between the fuel and a driver’s car.

"It opens up pretty much all of them," said Leon Markosian, of Leon's Auto Care in San Marino. "They're all standard keys."

Then came the "self-service" revolution. In the past few decades, the vast majority of California's 13,000 fuel stations converted to systems in which motorists can swipe their credit cards and pump their own gasoline.

Stations nationwide are now the same way. But the locks -- and those universal keys -- stayed largely the same.

And that's the problem, according to electronic fraud investigators, like Lt. Mike Sterner of the Santa Clara County District Attorney's office.

"A single key would open the credit card access door to any gas pump," he said.

Identity thieves have been obtaining and duplicating those universal keys from sources within the gasoline industry, police say, then sneak "skimmer" devices into the machines to harvest credit card information.

Some are so sophisticated that they only need to be installed once, wirelessly beaming information out to receivers nearby from positions deep inside the pumps' inner works.

The result? Maureen McGillan-Sklar, of Pasadena, is just one of thousands of Californians whose identities have been stolen, likely from gas station sources.

She started seeing mysterious charges on her monthly statements.

"They were all from Philadelphia," she said. "And a lot of them for gas."

Markosian is so attuned to the possibility of fraud at his San Marino gas station that he's taking no chances. His pumps are all full- or half-service. And there are no credit card swipers, just his trusted employees filling the tank and conducting the transactions.

Police recommend drivers use credit cards rather than debit cards at gas stations, because losses are absorbed by heavily-insured financial institutions.